1. Field of the Invention
This invention relates to a meter for measuring the suspended solids content in a liquid, and more particularly, to such meters for use in monitoring suspended solids content of a liquid which utilize sensing means for determining the transmission characteristics of a light beam passed through the liquid.
2. Description of the Prior Art
It is well established that a measurement of the concentration of solids suspended in a solution may be made by measuring the attenuation and/or scattering of a visible or near infrared light beam directed along a path extending through the solution. Optical windows are aligned with the path along which the light beam is directed. The major problem with this method of measurement is associated with keeping the optical windows free of light absorbing coatings deposited by the solution being monitored. Further, there is usually no way of knowing when the measurement system is providing satisfactory measurements or is producing erroneous indications due to meter malfunctions rather than change in the suspension conditions within the solution.
Several instruments are presently available which perform the functions of detecting and providing an indication of the quantity of particulate material suspended as solids in a solution. Schemes utilizing the effect of the suspended solids on the transmission of a light beam are the most common. The United States Environmental Protection Agency pamphlet, entitled Suspended Solids Monitor, by John W. Liskowitz and Gerald J. Franey, EPA-670/2-75-002, dated April 1975, discloses a device in which polarized light is directed into a solution containing suspended solids. The polarized light is scattered by the solids in suspension, and the amount of scattering is detected and processed to provide an indication of the suspended solids concentration. This scheme is more suitable for detection of a low concentration of suspended solids in solution and might more accurately be termed a turbidity meter.
Other forms of suspended solids detection apparatus are available using the light transmission principle, one of which includes a mechanical wiping structure for cleaning the optical windows along the light beam path through the solution. Such a device is manufactured by Biospherics Inc. of Rockville, Maryland as suspended solids meter Model 52LE. However, the accuracy of operation of such a device cannot be checked without removing the device from the site of operation and returning it to a laboratory for an accuracy test. As a consequence, the efficiency of the window cleaning function is unknown, and erroneous readings may be obtained if window cleaning is incomplete. No indication of the error is provided to those who utilize the obtained data.
A measurement of suspended solids in solution has also been made ultrasonically. A device utilizing such a system is manufactured by Nusonics, Inc. of Paramus, New Jersey and sold as a Sonic Solution Monitor. Such device comprises a transducer head and a receiver head which are immersed in a solution being monitored. The attenuation of the sound energy passed through the solution is measured to obtain an indication of the concentration of solids suspended in the solution. The ultrasonic method is subject to providing erroneous data without error indication if the solids in suspension change from one material to another or if changes occur in the associated electronic circuitry.
It is apparent that a suspended solids meter which retains its preset laboratory calibration characteristics throughout a prolonged period of usage, which provides an error indication when output data is unreliable, and which automatically recalibrates to assure continuous accuracy of data will be highly desirable to users of such devices.